In hydroponics the most widely use method of plant fertilization involves application of a nutrient. solution to the plant root. This is an effective method in most cases, but under certain conditions it may be quite ineffective. Though plant root systems can efficiently absorb the nutrients in normal circumstances, under conditions of root damage, nutrition deficiency or propagation from cuttings alternative strategies may need to be adopted. Foliar feeding, a technique of providing nutrients through the foliage of plants is often resorted to under such circumstances. Foliar feeding enables plants to absorb nutrients through the leaves. The minerals within the nutrients can then be translocated by plant within its tissues.



Nutrient Uptake in Hydroponics

Nutrient uptake, in hydroponics is a complex process that may be adversely affected by factors such as nutrient interactions, nutrient depletion, element unavailability due to the element being “bound" etc. Even environmental conditions like temperature can affect nutrient uptake through the roots. Plant pathogens such as fusarium pythium and phytophthora can also affect the normal functioning of the root zone severely restricting nutrient uptake. Other plant stress conditions such as anaerobic conditions in the root zone where oxygen is deficient, can limit nutrient uptake. Many other conditions can cause stress to plants such as humidity, lack of light, high radiation levels, etc. and directly or indirectly affect nutrient uptake. Under such situations foliar feeding has been found to be most effective in ensuring reliable nutrient uptake for healthy plant growth.

Foliar fertilization can thus help protect crop yields and quality against the vagaries of a wide variety of agents that can cause crop damage including climate, pests etc.

Foliar absorption

Absorption of nutrients in foliar feeding takes place through stomata on the leaves. These are located on the underside or on both sides of the leaf. The stomata normally function to enable gas exchange for photosynthesis and releasing water vapor in stomatal transpiration. But the leaf can also work as an organ for absorption and excretion of water and substances dissolved in it. Foliar feeding takes advantage of this to supply vital nutrients through the stomata. However, it is not possible for plants to be fed solely via the leaves; therefore the scope of foliar fertilization is limited. It is however, extremely effective as method of supplying micro nutrients.

There are several aspects of foliar fertilization that are not yet fully understood. There are several known and unknown factors that influence the effectiveness of foliar fertilization. It is known however, that the rate at which the nutrients supplied by the nutrient sprays are absorbed by the leaves and translocated within the plant is of critical importance. Also, practical experience provides several pointers for utilization of the technique for optimal benefit.

Application Considerations

The use of a good quality, non ionic wetting or sticking agents, such as Coco Wet, is vital to enable droplets to adhere to leaves. They also assist in the absorption of the fertilizer solution into the plant tissue. The foliar fertilizer solution is best applied as a fine mist until 'run off' so that the entire leaf surface is wetted. The effectiveness is also dependent on the timing of the application during the day. Early morning and evening are best suited as the conditions during these times of the day are ideal to allow the leaf to dry rather than stay wet for a long time. Foliar solutions are best applied while there is light but when temperatures are still cool; they should not be applied in hot, sunny conditions. In hot, sunny conditions the stomata are likely to be closed making the feeding ineffective. For the same reasons feeding is ineffective when the plants are wilting or under osmotic stress.

For the best results foliar feeding should be carried out on a regular, weekly basis. It is also found to be particular beneficial when limited to times of high nutrient demand. It is during stages of active growth such as during fruiting that foliar feeding is known to yield optimal results as during these stages leaves are particular efficient in absorbing nutrients.

Application in Hydroponics Cultivation

Even when hydroponic crops may appear to be well supplied with the necessary nutrients, it has been shown that they can still benefit from application of foliar fertilizers. Foliar fertilization was found in a number of studies involving hydroponically grown crops such as capsicum and potato, to dramatically increase yields. A weekly foliar feed applied to tomato crops grown in rockwool, also produced substantial improvements in both quality and quantitiy of the yields. It is believed that similar results can be obtained with a number of hydroponic crops with similar nutrient requirements. According to experts in the field, the process of foliar fertilization, is likely to emerge as a growth enhancing cultivation technique in the near future rather than just a 'quick fix' solution for mineral deficiency symptoms.